A wide variety of aircraft ducted fan systems have been experimented with for many years. A typical ducted fan system may include an inlet duct to direct an inflow of air to a rotating set of impellors situated downstream of an inlet face. The inlet duct may be contoured into a bellmouth profile that induces an internal duct pressure profile that is favorable for efficient thrust production at low speeds. This bellmouth profile, characterized by a relatively rounded inlet lip, may produce detrimental profile drag at high flight speeds, thereby reducing the aircraft's performance. To enhance high speed performance, a sharper inlet lip may be provided in place of the rounded inlet lip, but the sharper inlet lip fails to provide the favorable pressure profile needed for efficient thrust production at low flight speeds. Because of this need for rounded inlet lips in low speed flight and sharper inlet lips in high speed flight, ducted fan propulsion systems typically operate in a compromised state, with at least one important flight condition operating in a suboptimal state.
A variety of additional design features have attempted to overcome this fundamental limitation of ducted fan system design. In previous designs, movable louvers or doors may be incorporated to better control the airflow through the duct, but the curvature of the inlet lip remains fixed. In other existing designs, inflatable inlet surfaces integrated into the inlet lip may be deployed to provide a curved inlet lip profile at low flight speeds and may be deflated at high flight speeds to assume a sharp inlet profile. However, this inflatable inlet lip design may be vulnerable to rapid aging and to sudden failure with associated foreign object damage to the engine.
Ducted fan propulsion systems are particularly well-suited for use in VTOL (vertical takeoff and landing) aircraft due to their ability to produce high thrust at low flight speeds. In order to take off and land vertically, a VTOL aircraft must produce an amount of thrust that is greater than the aircraft's total weight and must further direct this thrust vertically downward at essentially zero flight speed. However, upon transition to horizontal flight, the propulsion system must produce thrust directed afterward relative to the fuselage of the aircraft at relatively high flight speeds.